1. Field of the Invention
The present invention relates to an air-micrometer calibration device.
2. Description of Related Art
Consider a case where, for example, a long drilling tool is attached to a main spindle of a machine tool, and a hole, such as a crank hole or a spool hole, which requires a strict coaxiality, is formed in a workpiece, such as a cylinder block or a valve body of an engine, by use of the drilling tool. In such a case, a bush is used for suppressing the vibration of the drilling tool. In the case of using a bush, the drilling tool is inserted into a bush hole with the axis of the main spindle (namely, the axis of the drilling tool) caused to coincide with the axis of the bush hole (see Part (a) of FIG. 1, the detail of which will be described later).
However, misalignment (eccentricity) sometimes occurs between the axis of the main spindle and the axis of the bush hole because of thermal deformation of the machine tool, and the like. If the drilling process is continued under such eccentric condition, the inner peripheral surface of the bush hole is unevenly worn. As a result, the bush fails to function properly, leading to degradation in the coaxiality of the machined hole.
Accordingly, the uneven wear of the bush hole needs to be prevented in the following manner. The amount of eccentricity between the main spindle and the bush hole is measured on a regular basis, and the position of the main spindle is controlled in accordance with the amount of eccentricity (that is, the relative position between the main spindle and the bush hole is corrected). Thereby, the axis of the main spindle (the drilling tool) and the axis of the bush hole are caused to coincide with each other.
In order to achieve this, a touch sensor has conventionally been used to measure the amount of eccentricity between a main spindle and a bush hole. Part (a) of FIG. 21 is a side view of a touch sensor, and Part (b) of FIG. 21 is a view in the direction of the arrow W in Part (a) of FIG. 21. As shown in these figures, a touch sensor 1 includes a measuring head 2 and a stylus 3 protruding on the tip of the measuring head 2. The measuring head 2 is attached to a main spindle 4 of a machine tool in place of a drilling tool. Thereafter, the main spindle 4 is operated to bring a stylus ball 3a on the tip of the stylus 3 into contact with the inner peripheral surface of a bush hole (illustration of which is omitted), whereby the amount of eccentricity between the main spindle 4 and the bush hole is measured.
However, the conventional touch sensor has the following problems because it is a contact sensor, and so on.    (1) A measurement error is likely to occur because of the biting of foreign matter, such as a chip, attached to the inner peripheral surface of a bush hole.    (2) Since the stylus 3 is easily broken, the main spindle needs to be operated at a low speed in order to prevent the breakage of the stylus 3, so that the measurement takes a long time.    (3) Every time the measuring head 2 is replaced because of the failure of the stylus 3, or the like, it is necessary to perform calibration using a dial gauge, which increases the time taken for the measurement.
On the other hand, an air micrometer has been known as a non-contact sensor, which enables measurement in a short time and at high accuracy. Part (a) of FIG. 22 is a view showing the outline of a conventional air micrometer, and Part (b) of FIG. 22 is a view showing the outline of a calibration device for the air micrometer.
As shown in Part (a) of FIG. 22, a measuring head 11 of the conventional air micrometer includes a measuring-head body portion 14 and a measuring-head tip portion 12 formed on the distal end of the measuring-head body portion 14. In the measuring-head tip portion 12, a first measurement air nozzle 16A and a second measurement air nozzle 16B are formed to extend respectively in the opposite directions to each other along the radial direction of the measuring-head tip portion 12. In the measuring-head body portion 14, a measurement air supply passage 15 communicating with the first measurement air nozzle 16A and the second measurement air nozzle 16B is formed.
In the measurement, after the measuring head 11 (the measuring-head tip portion 12) is inserted into a hole 13a of a measurement target 13 as illustrated, measurement air is supplied from a measurement air supply source 17, through an A/D converter 18, to the measurement air supply passage 15 in the measuring-head body portion 14. After passing through the measurement air supply passage 15, the measurement air is divided into two flows, which are thus jetted from the first measurement air nozzle 16A and the second measurement air nozzle 16B, respectively. In this event, the A/D converter 18 detects the pressure of the measurement air (which corresponds to the flow rate of the measurement air), converts the detection signal into a digital signal, and outputs the digital signal to a control device (illustration of which is omitted). The control device obtains the flow rate of the measurement air from the pressure detection signal outputted from the A/D converter 18, and obtains the diameter D1 of the measurement target hole 13a on the basis of data on the flow rate of the measurement air and pre-stored data representing the relationship between the hole diameter and the flow rate of the measurement air.
In addition, the data representing the relationship between the measurement flow rate and the hole diameter is obtained in advance by use of an air-micrometer calibration device (a master gauge) 19 as shown in Part (b) of FIG. 22. Specifically, after the measuring head 11 (the measuring-head tip portion 12) is inserted into a master hole 19a, having a predetermined diameter D2, of the air-micrometer calibration device 19 as illustrated, the measurement air is supplied from the air supply source 17, through the A/D converter 18, to the air supply passage 15 in the measurement-head main body 14. After passing through the measurement air supply passage 15, the measurement air is divided into two flows, which are thus jetted from the first measurement air nozzle 16A and the second measurement air nozzle 16B, respectively. The A/D converter detects the pressure of the measurement air (which corresponds to the flow rate of the measurement air) in this event, converts the detection signal into a digital signal, and outputs the digital signal to the control device (not illustrated). The control device obtains the flow rate of the measurement air from the pressure detection signal outputted from the A/D converter 18. This measurement is performed on two types of master holes having different diameters D2, that is, large and small master holes 19a. Then, the control device stores data on the flow rate of measurement air and data on the diameter D2 inputted in advance as the aforementioned data representing the relationship between the flow rate of the measurement air and the hole diameter.    Patent Document 1: Japanese Patent Application Publication No. 2006-284376    Patent Document 2: Japanese Patent Application Publication No. Sho 58-114835    Patent Document 3: Japanese Patent Application Publication No. Hei 6-186009    Patent Document 4: Japanese Patent Application Publication No. Hei 7-134018